1. Field of the Invention
This invention relates to fusion processes for producing sheet glass and, in particular, to fusion processes which employ a crystalline ceramic isopipe. Even more particularly, the invention relates to controlling the formation of crystalline defects in sheet glass produced by fusion processes employing ceramic containing isopipes. The techniques of the invention are particularly useful when fusion processes are employed to produce glass sheets for use as substrates in the manufacture of liquid crystal displays, e.g., AMLCDs
2. Technical Background
The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of liquid crystal displays (LCDs).
The fusion process, specifically, the overflow downdraw fusion process, is the subject of commonly assigned U.S. Pat. Nos. 3,338,696 and 3,682,609, to Stuart M. Dockerty. As described therein, molten glass is supplied to a trough formed in a refractory body known as an “isopipe”.
In an exemplary fusion downdraw process as described in the Dockerty patent, once steady state operation has been achieved, molten glass overflows the top of the trough on both sides so as to form two half sheets of glass that flow downward and then inward along the outer surfaces of the isopipe. The two sheets meet at the bottom or root of the isopipe, where they fuse together into a single glass sheet. The single sheet is then fed to drawing equipment which controls the thickness of the sheet by the rate at which the sheet is drawn away from the root. The drawing equipment is located sufficiently downstream of the root so that the single sheet has cooled before coming into contact with the equipment.
The outer surfaces of the final glass sheet do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces see only the ambient atmosphere. The inner surfaces of the two half sheets which form the final sheet do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.
An isopipe used in the fusion process is subjected to high temperatures and substantial mechanical loads as molten glass flows into its trough and over its outer surfaces. To be able to withstand these demanding conditions, the isopipe is typically and preferably made from an isostatically pressed block of a refractory material (hence the name “iso-pipe”). In particular, the isopipe is preferably made from an isostatically pressed zircon refractory, i.e., a refractory composed primarily of ZrO2 and SiO2. For example, the isopipe can be made of a zircon refractory in which ZrO2 and SiO2 together comprise at least 95 wt. % of the material, with the theoretical composition of the material being ZrO2.SiO2 or, equivalently, ZrSiO4.
A source of losses in the manufacture of sheet glass for use as LCD substrates is the presence of zircon crystal inclusions (referred to herein as “secondary zircon crystals” or “secondary zircon defects”) in the glass as a result of the glass' passage into and over the zircon isopipe used in the manufacturing process. The problem of secondary zircon crystals becomes more pronounced with devitrification-sensitive glasses which need to be formed at higher temperatures.
Zircon which results in the zircon crystals which are found in the finished glass sheets has its origin at the upper portions of the zircon isopipe. In particular, these defects ultimately arise as a result of zirconia (i.e., ZrO2 and/or Zr+4+2O−2) dissolving into the molten glass at the temperatures and viscosities that exist in the isopipe's trough and along the upper walls (weirs) on the outside of the isopipe. The temperature of the glass is higher and its viscosity is lower at these portions of the isopipe as compared to the isopipe's lower portions since as the glass travels down the isopipe, it cools and becomes more viscous.
The solubility and diffusivity of zirconia in molten glass is a function of the glass' temperature and viscosity (i.e., as the temperature of the glass decreases and the viscosity increases, less zirconia can be held in solution and the rate of diffusion decreases). As the glass nears the bottom (root) of the isopipe, it may become supersaturated with zirconia. As a result, zircon crystals (i.e., secondary zircon crystals) nucleate and grow on the bottom portion (e.g. root) of the zircon isopipe. Eventually these crystals grow long enough to break off into the glass flow and become defects at or near the fusion line of the sheet. Moreover, if the temperature of the glass at the isopipe root is too low, devitrification of the glass may occur. Thus, it is desirable to increase the temperature of the isopipe near the isopipe root. Unfortunately, raising the temperature near the root of the isopipe has had the unpleasant effect of also increasing the temperature of the molten glass within the isopipe trough, decreasing the viscosity of the glass and hence impacting the mass flow distribution of the glass. This change in mass flow distribution can be compensated by tilting the isopipe, but only within a narrow range of angles. Heating at the top of the isopipe occurs because the heating elements typically used to modify the temperature of the glass flowing down the sides of the isopipe are contained within a common plenum. As illustrated in FIG. 1, an isopipe 10, as commonly used today, comprises a plurality of heating elements 12a-12d and 14a-14d distributed upward from root 14 along both sides of the isopipe. Heating elements 12a-12d and 14a-14d are contained within the structure of enclosure 16 and more particularly within common plenum 18. As a result, an increase in the temperature of the bottom-most heating element has a noticeable effect on the temperature at the top of isopipe 10.